In recent years, a renewable energy power generation device for generating electric power from wind power, solar photovoltaic energy, geothermal energy, or the like has attracted attention and been put to practical use. The renewable energy power generation device is a clean power generation device that does not use any limited resource such as oil but uses an energy resource present in unlimited quantity in nature, and can suppress carbon dioxide emissions. Hence, the renewable energy power generation device has been increasingly introduced by companies, governments, and the like worldwide, for prevention of global warming.
However, since energy coming from nature varies from moment to moment, output fluctuations are unavoidable in the renewable energy power generation device. This poses an obstacle to widespread use of the renewable energy power generation device.
To remove this obstacle and achieve constant output, in the case of employing the renewable energy power generation device, an interconnected (power generation) system that combines the renewable energy power generation device with an electric power storage-compensation device including a secondary battery as a main component is built.
As one type of the secondary battery, a sodium-sulfur battery has features such as a high energy density, an ability to produce high output in a short time, and an excellent high-speed response. Accordingly, by adding a bidirectional converter for controlling charge and discharge, it is possible to compensate for output fluctuations of the renewable energy power generation device which can occur on the order of several hundred milliseconds to several seconds. Therefore, the interconnected system in which the renewable energy power generation device is combined with the electric power storage-compensation device that includes the sodium-sulfur battery as a component is a desirable power generation system.
Though there seems to be no prior art that has the same object as the present application, relevant techniques are disclosed, for example, in. Patent Document 1 (JP-A-2003-317808) and Patent Document 2 (JP-A-2008-084677).